1. Field of the Invention
This invention relates generally to motor control systems and specifically to motor control systems for providing adequate starting torque and facilitating motor efficiency especially in low speed.
2. Description of the Background Art
Electric motors typically include a stator, which is wound with start windings and run windings connected to the source of power. The stator windings surround a rotor which rotates a shaft to produce the motor output. Rotors are made in a number of different configurations which are well known in electric motor industry.
Conventional single phase motors commonly have a multi-speed capability and frequently are constructed in a capacitor start or a capacitor start/capacitor run configuration. In capacitor start motors, start circuits are connected in parallel with the run winding of each speed of the multi-speed motor. The start circuits include start windings which are connected in series with start capacitors. During a motor start, both the run windings and the start windings are connected across the motor power source to magnetically excite the rotor and cause rotation thereof. The start winding and start capacitor combination is used to provide the high torque required during typical start conditions. However, the capacitance necessary to start the motor is typically too large for optimal motor efficiency. Therefore, a switch is usually connected in series with the start capacitor which is used to disengage the start circuit when the motor has either reached a predetermined speed or after a predetermined time has elapsed. However, once the start circuit is disengaged the motor still does not operate at maximum efficiency.
In capacitor start/capacitor run motors, run capacitors are used in addition to the start circuits for each speed of the multi-speed motor. The run capacitors are connected in series with a secondary windings and both are connected in parallel with the run windings for each speed of the multi-speed motor. The motor windings and the run capacitors in this configuration are tuned to run at optimal efficiency as is know in the art. Thus, when the start circuits are disengaged, the motor can be designed to run at optimal efficiency. However, this configuration requires additional complexity, costs, and potential for failure of such motors.
Optimal motor efficiency is an important criteria in designing refrigeration systems. For example, certain applications of refrigeration systems require low speed motor operation at low temperature ranges where the refrigeration system has a low capacity. Low efficiency in the low speed setting of the refrigeration system causes excessive motor heat to be added to the evaporator load which in turn further reduces the system's cooling capacity. Costs and reliability also are important criteria in designing refrigeration systems.
Thus, it is desirable to have a multi-speed motor which is low cost and simple in design while providing improved efficiency at low speeds.